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Amtliche Mitteilungen der Technischen Universität Dortmund, Nr. 5/2026
(Technische Universität Dortmund, 2026-02-20)
Frictional behaviour of coated carbide tools and AISI 316L when using translational and rotatory relative movement considering dry and lubricated conditions
(2024-04-12) Volke, Pascal; Courbon, Cédric; Krumme, Erik; Saelzer, Jannis; Rech, Joel; Biermann, Dirk
In machining, tool temperatures and thus tool wear are significantly influenced by frictional behaviour. Friction tests are used to determine the friction coefficient depending on relative speed, which serves as basis for parameterising friction models as input data for chip formation simulations. Therefore, this paper represents investigations towards the frictional behaviour of uncoated and coated (TiN, TiAlN) carbide tools when using two different relative movements (translational and rotary) and cooling lubricant conditions. In dry conditions, the investigations show insignificant influence of different engagement surfaces and testing kinematics on resulting friction. In lubricated conditions, three different friction coefficient sections were observed.
From frequency-dependent models to frequency-independent enriched continua for mechanical metamaterials
(2024-02-23) Rizzi, Gianluca; d'Agostino, Marco Valerio; Voss, Jendrik; Bernardini, Davide; Neff, Patrizio; Madeo, Angela
Mechanical metamaterials have recently gathered increasing attention for their uncommon mechanical responses enabling unprecedented applications for elastic wave control. Many research efforts are driven towards the conception of always new metamaterials’ unit cells that, due to local resonance or Bragg-Scattering phenomena, may produce unorthodox macroscopic responses such as band-gaps, cloaking, focusing, channeling, negative refraction, etc. To model the mechanical response of large samples made up of these base unit cells, so-called homogenization or upscaling techniques come into play trying to establish an equivalent continuum model describing these macroscopic metamaterials’ characteristics. A rather common approach is to assume a priori that the target continuum model is a classical linear Cauchy continuum featuring the macroscopic displacement as the only kinematical field. This implies that the parameters of such continuum models (density and/or elasticity tensors) must be considered to be frequency-dependent to capture the complex response of the considered mechanical systems in the frequency domain. These frequency-dependent models can be useful to describe some of the aforementioned macroscopic metamaterials’ properties, yet, they suffer some drawbacks such as featuring negative masses and/or elastic coefficients in some frequency ranges which are close to resonance frequencies of the underlying microstructure. This implies that the considered Cauchy continuum is not positive-definite for all the considered frequencies. In this paper, we present a procedure, based on the definition of extra kinematical variables (with respect to displacement alone) and through the use of the inverse Fourier transform in time, to convert a frequency-dependent model into an enriched continuum model of the micromorphic type. All the parameters of the associated enriched model are constant (i.e., frequency-independent) and the model itself remains positive-definite for all the considered frequency ranges. The response of the frequency-dependent model and the associated micromorphic model coincide in the frequency domain, in particular when looking at the dispersion curves. Moreover, the micromorphic (frequency-independent) model results to be well defined both in time- and in the frequency-domain, while the Cauchy (frequency-dependent) model can only exist in the frequency domain This paper aims to build a bridge between the upscaling techniques usually found in the literature and our persuasion that macroscopic continua of the micromorphic type should be used to model metamaterials’ response at the macroscopic scale.
Generation of compressed air by overdriven free-displacer thermocompressors
(2024-08-15) Fischer, Fabian; Kühl, Hans-Detlev
Due to their straightforward design and advantageous operational characteristics, cascaded arrangements of overdriven free-displacer thermocompressors may provide a simple, expedient and cost-effective solution to reduce the high primary energy requirements for compressed air generation, which is a common challenge in industry. Following first experiments with a single-stage prototype, this contribution presents the first experimental realization of a three-stage cascade so that the overall concept has now been demonstrated to be fully operational for the first time. In particular, the predicted self-control capabilities of a stage located within the cascade and thus without any preset inlet or outlet pressure could be proven, whereas this had been impossible with the single-stage prototype before. Stable operation with all stages running is possible at total pressure ratios ranging from a practical maximum near 2.1 down to approximately 1.36. At even lower values, the pressure ratio of the first stage undercuts an analytically predicted lower stability limit and eventually stops, whereas the power density of the remaining stages is increased. To exploit the potential for improvement identified during the single-stage experiments, the design of the new stages is based on the first prototype, but has been optimized by a similarity-based scaling procedure and design evolution, resulting in a significant increase in power density from 7.1 W to 12.5 W per liter of displacement. To further demonstrate the capability of such a cascade to extract waste heat from a hot stream, the heater temperatures of the stages were successively reduced along the cascade by an adequate control, thus emulating a finite heat capacity rate of the stream. Although the exergy transferred to the compressed air flow is reduced in this case, the overall performance is not adversely affected. The combination of the models and the scaling approach, which have now been demonstrated experimentally, provides a comprehensive toolbox for the future development of a series machine.
Generation of compressed air by overdriven free-displacer thermocompressors – experimental investigation of a single stage
(2024-02-13) Fischer, Fabian; Kühl, Hans-Detlev
A thermocompressor cascade of identical stages has been identified as a promising approach to utilize waste heat for the direct generation of compressed air. This contribution presents the first design, realization and experimental investigation of a single-stage prototype featuring a free displacer oscillating in overdriven mode, which has so far only been devised and analyzed theoretically and by simulations. It is self-excited by the p,V-work generated by its rod, which periodically plunges into the cold cylinder volume. In general, the experimental performance confirms the expectations, as a stable, self-controlled operation is possible within a wide range of pressure ratios and inlet pressure levels. However, the theoretical maximum pressure ratio, at which the machine would inevitably stop, is not reached experimentally. Instead, operation continues at a lower pressure ratio, where the net mass flow drops to zero due to leakage effects. This behavior could be confirmed by an enhanced numerical model. In addition, an automatic, self-excited start by decreasing the pressure ratio could be experimentally confirmed. This is a typical industrial operating scenario, when compressed air is fed to a reservoir and consumption increases. These promising findings strongly suggest the realization and testing of a multi-stage cascade as the next step.